I believe in this day and age it's called 'micro-cheating'. I'd go as far as to say it went beyond emotionally cheating in this scenario as well because she intended to go and have actual sex with him at some point down the line.

Don't feel bad for looking at her phone. People have been 'getting caught out' for centuries. It's the stereotypical response in this day and age, after being caught out, to go "Errmergerd you were looking at my phone?!" It's just her way of deflecting a part of her awful behaviour onto you.

Privacy and trust can only go so far in my opinion. And if she's giving you reason to feel mistrusting, then what did she expect. *shrugs*